In the quest for hydrocarbon reservoirs, companies employ many data-gathering techniques. The most detailed, albeit localized, data comes from well logging. During the well-drilling process, or shortly thereafter, instruments are passed through the well bore to collect information about the formations through which the well bore passes. The information is traditionally collected in “log” form, i.e., a table or chart of measured data values as a function of instrument position. The most sought-after information relates to the location and accessibility of hydrocarbon gases and fluids.
Resistivity, density, and neutron porosity logs have proven to be particularly useful for determining the location of hydrocarbon gases and fluids. These logs are “open hole” logs, i.e., the log measurements are taken before the formation face is sealed with tubular steel casing. Resistivity can be measured in a number of ways that are not important here. Density is traditionally measured by determining the scattering and absorption of gamma rays emitted from a radioisotopic gamma ray source. Neutron porosity is commonly measured by determining the scattering of neutrons from hydrogen nuclei in the formation. (Neutron porosity is primarily a measure of hydrogen concentration, and hydrogen predominately appears in fluids contained in the formation pores.) Neutron porosity and density measurements may be combined to provide improved estimates of formation porosity.
The neutrons for the porosity measurement are relatively low energy neutrons supplied by a radioisotopic neutron source. As used herein, the term “radioisotopic source” refers to those sources of alpha particles, beta particles, gamma rays, or neutrons, that depend on the natural decay of a radioactive isotope. Cesium 137 is an example of a radioisotopic source that produces beta particles and gamma rays. Americium 241 is an example of a radioisotopic source that produces alpha particles and gamma rays. Another example of a radioisotopic source is the combination of americium 241 with beryllium 9. When the beryllium absorbs an alpha particle from the americium decay, a carbon atom is formed and a neutron is emitted. (Because this radioisotopic source relies on a combination of elements, it is sometimes referred to as a chemical source.) Thus the Am/Be source emits alpha particles, gamma rays, and neutrons. The Am/Be source is primarily used for neutron porosity measurements, but because the neutron interactions with formation nuclei generate secondary gamma rays, the Am/Be source can also be used for density logging measurements that rely on gamma ray attenuation.
Radioisotopic sources present certain risks to human health and they may be a primary ingredient in weapons of terrorism. Even in routine field operations, the involved oilfield workers encounter radiation exposure risks from the use of these sources. When exposed to sufficient radiation from such sources, humans experience cellular damage that can cause cancer and (at higher doses) radiation sickness. These adverse health effects are often lethal. The source materials described above have long half-lives (30 years for cesium 137, and 5300 years for americium 241), meaning that the radiation from these sources will persist for a very long time if they should be accidentally or intentionally dispersed into the environment.
It should come as no surprise, then, to discover that the government heavily regulates the possession and transportation of radioisotopic sources. See, e.g., 10 CFR Part 1-Part 1060 (regulations from the NRC and DOE) and Federal Register vol. 70, no. 44, Jul. 28, 2005 (Proposed rule changes to 10 CFR Parts 20, 32, and 150, concerning the NRC National Source Tracking Database). Such regulations impose considerable costs for establishing and maintaining compliance. Despite such regulations, the authors are given to understand that on average, at least one such radioisotopic source is misplaced or stolen each year. See, e.g., Russell Gold and Robert Block, “Radioactive Material Is Stolen From Halliburton”, Mar. 6, 2003 (discussing the theft of a radioisotopic source and the dangers of a dirty bomb).
In addition, extensive safety procedures are needed to protect workers who transport, store, and use radioisotopic sources. Radiation from such sources can produce heat, ionization, and chemical changes which lead to corrosion of storage containers. Regular “wipe” tests are conducted to monitor sources for leakage, radiation sensors are put into storage facilities to monitor radiation levels, and employees are given radiation-sensitive badges to monitor employee exposure levels. Cumulatively, the tests, monitoring equipment, transportation, and storage facilities present a severe budgetary impact to any company that employs such sources.
Moreover, when compliance efforts are combined with necessary safety procedures, the result is a considerable effort and delay in getting a radioisotopic source to the location in the field where it is needed. To further compound the problem, the preferred radioisotopic sources are in short supply. The largest supplier of americium 241 was the US Department of Energy, which had accumulated a stockpile of this material from various refining operations on other radioactive materials. These stockpiles have now been exhausted, and currently the only continuing source of this material is an aging breeder reactor in eastern Europe.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereof are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.